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much as the force, acting in a radius from a centre, would be 

 more free to exert itself horizontally than otherwise. 



Dr. Charles T. Jackson stated that he had several times, dur- 

 ing his geological survey of the State of Maine, examined the 

 slide on the Presumpscot River, in Westbrook, and had published 

 a short description of it in his annual reports on that survey. 



He agreed with Mr. Bouve in opinion that an undermining of 

 the strata of clay, by the action of the river, probably induced the 

 slide, but he was satisfied that the strata of clay marl had slid 

 forward toward the river as well as fallen from the original level 

 of the remaining bank. Whether the frosts of winter cracked 

 the clay at its junction with the bank now standing, and thus let 

 in the water to the soft, plastic clay below, rendering it very 

 slippery, or the desiccation of it had caused such a fissure, was 

 unknown, but there cannot be a doubt that it was owing to the 

 extreme plasticity of the under blue clay that the strata were 

 enabled to slide toward the river, as they have done. 



The positions of the fir-trees upon the clay bed prove a sliding 

 motion of the mass toward the river. 



With regard to the concretions found at the slide. Dr. Jack- 

 son stated that the clay marl in which they are found contains 

 about ten per cent, of carbonate of lime, while the concretions 

 generally contain as much as 50 per cent. He considered the 

 crystallizing force of the cai'bonate of lime to be the cause of the 

 concretionary structure and form of these bodies, the foreign 

 bodies occasionally found Avithin them serving as nuclei around 

 which this semi-crystallization took place, the carbonate of lime 

 segregating and carrying with it the inert particles of clay, the 

 spheroidal form being that which would result from this action 

 when the force was not adequate to the production of crystals. 

 He illustrated this view by reference to the spheroidal struc- 

 ture of hyalite and of various hydrous silicates which form from a 

 gelatinous paste in which there is not sufficient freedom of motion 

 to allow of the formation of perfect crystals. In case there were 

 a larger proportion of carbonate of lime in solution as a bicar- 

 bonate, the crystalline foi'ms would become more perfect, as in 

 the well-known crystallized sandstone of Fontainbleau, in which 

 grains of silicious sand are forced into the form of calcareous 



